Mass marking for spectrometry using programmed molecule clusters

ABSTRACT

Mass marking in mass spectrometry is accomplished by molecule clusters having molecular weights at regular mass intervals over a mass range within which ions of a gas to be analyzed are expected. A marker gas (such as argon) is employed which is capable of forming molecule clusters in a controlled expansion through a nozzle into a region of reduced pressure. By preselecting or programming the source pressure of the selected marker gas, the intensity of the molecule clusters is optimized over the mass range of interest. A beam of such clusters is formed by the nozzle, and is directed into the ion source of a mass spectrometer. A fast acting valve at the nozzle may be employed to provide intermittent discharge of the marker gas, thereby reducing the gas loads on the vacuum pumping systems. For phase sensitive detection, a mechanical chopper in the path of the beam is utilized to modulate the beam to impart a characteristic signature to the ionized clusters.

United States Patent [191 Milne Apr. 2, 1974 MASS MARKING FOR SPECTROMETRY USING PROGRAMMED MOLECULE CLUSTERS [75] Inventor: Thomas A. Milne, Quivira, Kans.

[73] Assignec: Midwest Research lnstitute, Kansas City, Mo.

[22] Filed: I Nov. 16, 1972 [21] Appl. No.: 307,085

[52] US. Cl. 250/4l.9 ME, 250/41.9 S, 250/4l.9 D [51] Int. Cl. H0lj 39/34, BOld 59/44 [58] Field of Search 250/419 G, 41.9 S, 41.9 ME,

[56] References Cited FOREIGN PATENTS OR APPLICATIONS 1,147,651 4/1969 Great Britain... 250/419 D Primary Examiner.lam'es W. Lawrence Assistant ExaminerC. E. Church Attorney, Agent, or Firm-D. A. N. Chase [57] ABSTRACT Mass marking in mass spectrometry is accomplished by molecule clusters having molecular weights at regular mass intervals over a mass range within which ions of a gas to be analyzed are expected. A marker gas (such as argon) is employed which is capable of forming molecule clusters in a controlled expansion through a nozzle into a region of reduced pressure. By preselecting or programming the source pressure of the selected marker gas, the intensity of the molecule clusters is optimized over the mass range of interest. A beam of such clusters is formed by the nozzle, and is directed into the ion source of a mass spectrometer. A fast acting valve at thenozzle may be employed to provide intermittent discharge of the marker gas, thereby reducing the gas loads on the vacuum pumping systems. For phase sensitive detection, a mechanical chopper in the path of the beam is utilized to modulate the beam to impart a characteristic signature to the ionized clusters.

13 Claims, 6 Drawing Figures PATENIEUMR 2 i974 PROGRAMMABLE CONTROL sum 10? 2 ASS MASS MARKING FOR SPECTROMETRY USING PROGRAMMED MOLECULE CLUSTERS This invention relates to improvements in mass marking in mass spectrometry and, more particularly, to a method and apparatus by which regular mass intervals are marked over a mass range of interest utilizing molecule clusters of a marker gas.

Presently available mass markers for organic mass spectrometry applications, of which perfluoro kerosene is the most popular, suffer severe limitations. One of the most serious of these is the lack of intense ions at high masses, particularly the region of growing importance beyond 750 amu. In the case of perfluoro kerosene, this compound has a discontinuous mass spectrum in addition to being a poor marker at high masses.

Heretofore, it has been required that amass marker be both volatile and have a molecular structure that would yield mass markings in the range of interest. Accordingly, workers in spectrometry were limited to the selection of heavy, organic compounds with the inherent molecular marking limitations characteristic of the selected compound. In short, the nature of the marker gas was restricted to the chemistry of the compound, thus markings at high atomic weights could not be achieved without selecting a heavy compound. A further complication has been in the' tailoring of the marker gas to the sample gas to be analyzed in order to avoid overlapping peaks in the spectrometer readout which would degrade the resolution.

It is, therefore, an important object of the present invention to provide an improved method of marking molecular weights over a mass range within which ions of a gas to be analyzed by mass spectrometry are expected in order to provide a reference for identification of the ions, wherein such method is not subject to the disadvantages and limitations discussed above.

As a corollary to the foregoing object, it is an important aim of this invention to provide a method as aforesaid wherein the marking of molecular weights is effected at regular mass intervals over the mass range of interest, utilizing a marker gas having a relatively low molecular weight.

Another important object of the present invention is to provide a method as aforesaid employing a marker gas having molecules of known molecular weight and characterized by the capability of forming molecule clusters at regular mass intervals in a controlled expansion of the marker gas, and wherein the formation of such .clusters enables very high mass ranges to be at tained by the occurrence of significant numbers of clusters at the successive mass intervals.

Still another important object of this invention is' to provide a method as aforesaid employing molecule clusters which yield atomic weights well in excess of mass 1,000 at predictable and regular mass intervals.

Still further, it is an important object of this invention to provide a method as aforesaid wherein, by preselecting or programming the source pressure of the selected marker gas, the intensity of the molecule clusters may be optimized over the mass range-of interest.

Additionally, it is an important aim of the present invention to provide apparatus for implementing the aforesaid method and, in particular, for use in conjunction with existing mass spectrometer equipment.

Yet another important object of this invention is to provide a method and an apparatus for mass marking with molecule clusters, wherein the gas loads on the vacuum pumping systems are minimized by intermittent delivery of the marker gas to the nozzle from which the expanded gas is discharged.

Another important object of this invention is to provide a method and an apparatus for mass marking with molecule clusters wherein a unique signature is -imparted to the clusters by modulation of the cluster beam.

Further objects and advantages of the present invention include the ability to choose a marker gas having a large negative mass defect, the ability to provide marker distributions that are stable and reproducible in intensity, and the ability to provide a marker gas which is entirely noncontaminating and noncorrosive.

In the drawings:

FIG. 1 is a partially diagrammatic and schematic illustration of the apparatus of the present invention, showing the conduit thereof in longitudinal cross section communicating with a port of the ion source of a mass spectrometer;

FIG. 2 is a block diagram of a control arrangement for varying the source pressure of the marker gas;

FIG. 3 is an enlarged view of one face of the chopper disc of the mechanical modulator illustrated in FIG. 1;

FIG. 4 is a direct reproduction of a strip chart recording of a scan of argon clusters over the mass range from 1,160 to 1,400 amu;

.FIG. 5 is a graph illustrating the clustering of argon atoms at three different source pressures; and

FIG. 6 is a graph illustrating the distribution of molecule clusters of argon for three different nozzle configurations.

SUMMARY AND BACKGROUND In the present invention, neutral molecule clusters are generated in an adiabatic expansion of a marker gas and, after ionization, are employed as mass markers in a mass spectrometer (or a mass spectrograph). As used herein, the terms spectrometry and spectrometer" refer to the analysesof mass spectra irrespective of the mode of readout (electrical or photographic) employed in the mass analyzing apparatus. The selection of the marker gas is dependent upon the mass range within which ions of a gas to be analyzed are expected. The rare gases (particularly neon and argon) are especially suitable for use as the marker gas since molecule clusters at desirable mass intervals over any practically useful mass range can be readily generated. The molecule clusters are formed as a beam discharging from a nozzle under conditions causing a controlled expansion, and the beam is directed into the ion source of the spectrometer. By preselecting the source pressure of nism of a controlled expansion. The rare gases, nitro-- gen and carbon dioxide are particularly attractive because of their low molecular weight, favorable expansion properties and simple cluster spectra. In recent literature, I-Iagena and Obert disclose their fingings regarding control of the extent of cluster formation by variation of the type of gas, the starting temperature (prior to expansion), the source pressure, and the nozzle configuration. (See I-Iagena, O. Fpand W. Obert, Cluster Formation in Expanding Supersonic Jets: Effect of Pressure, Temperature, Nozzle Size and Test Gas. .1. Chem. Phys., Vol. 56, Number 5, p. 1793 (1972).) Also, Buchheit and I-Ienkes have generated hydrogen clusters using nozzle expansion and have suggested that H clusters would theoretically provide calibration masses usable in mass spectrometry. (See Buchheit, K. and W. Henkes, Zeits, fur Angewandte Physik, Vol. 24, Number 14, p. 191 (1968).)

DETAILED DESCRIPTION OF THE APPARATUS Referring initially to FIGS. l-3, a two-stage vacuum system is illustrated having a conduit which is in communication with a port 12 ofthe ion source 14 of a mass spectrometer. The right end of the conduit 10 presents the outlet thereof which is registered with the port 12. It may be appreciated that the conduit 10 and port 12 are coaxially arranged for the purpose of directing a beam 16 through the ionizing field 18 of the source 14-, as will be explained.

A supplyextends 20 is coaxially arranged within the conduit 10 and estends thereinto from the left as viewed in FIG. 1, the end of the pipe 20 being formed with a restriction 22. A fast acting valve is illustrated adjacent the end of the pipe 20, and comprises a coil 24 which, upon energization thereof, attracts an armature 26 and shifts the same to the left against the bias of a return spring. The armature 26 is shown partially withdrawn for purposes of illustration, and is provided with a conical tip that. mates with the converging seat presented by. the restriction 22.

The end of the pipe 20 is fitted with a special nozzle 28 which is aligned with the restricted opening, as is apparent in FIG. 1. For purposes ofillustration, the nozzle 28 is shown as being of conical configuration. The significance of the shape of the nozzle will be discussed hereinafter.

The operating coil 24 of the valve is connected by a pair of leads 30 to a pulser 32 that enables the apparatus to operate on a timed duty cycle to minimize gas load. Accordingly, the pulser 32 may comprise an electrical timing circuit having an .output which delivers time-spaced electrical pulses to cause intermittent operation of the valve. Alternatively, pulse generation could be synchronized with the scan speed of the spectrometer.

A wall 34 in the conduit 10 separates the first and second stages of the vacuum system. An inlet pipe 36 ofa vacuum pump (not shown) communicates with the first stage of the system, and an inlet pipe 38 of a second vacuum pump communicates with the second stage. Exemplary pressures are 10 to 10 torr in the first stage and 10' torr in the second stage. Accordingly, the two-stage system provides for differential pumping and is useful for purposes of collimation of the beam 16 to be discussed.

The marker gas is supplied via the pipe 20 and is under pressure (two to five atmospheres for example), thus expansion of the gas occurs as it passes through the nozzle 28 into the region of reduced pressure established within the first stage. The expansion is adiabatic and causes partial condensation of the marker gas and attendant formation of the beam 16, which is a beam of molecule clusters having molecular weights at integer multiples of the molecular weight of the marker gas monomer. A cone-shaped skimmer 40 extends toward the nozzle 28 from the wall 34 and, in the second stage, a cone-shaped collimator 42 in the conduit outlet further collimates the beam 16 as it enters the port 12 of the ion source 14. Three other ports of the ion source 14 are also illustratedport 44 for receiving the sample gas, port 46-from which ions are discharged into the analyzer (not shown) of the spectrometer, and port 48 which is communicated with the vacuum pumping system employed to maintain the ion source chamber in an evacuated condition.

In some instances it may be desired to modulate the beam 16. For this reason, a mechanical modulator is illustrated in the second stage of conduit 10 and comprises an electric motor driving a chopper disc 52. The configuration of the disc 52 may be seen in FIG. 3 in relation to the beam 16. A pair of diametrically opposed apertures 54 permit the beam 16 to pass uninterrupted whenever either aperture 54 is aligned with the beam. Each aperture 54 encompasses a arc and is followed by a 90 span of uninterrupted disc, thus square wave intensity modulation is imparted to the beam entering the port 12. The modulation frequency would be very high relative to the scan rate of the spectrometer, and thus could be on the order of 100 Hz to as high as 10,000 Hz.

FIG. 2 illustrates the programming of the pressure of the marker gas in the supply pipe 20. A pressure regulator valve 56 is interposed in the pipe 20 shown extending from the gas source 58. A suitable sensing device such as a Hall probe 60 delivers a direct current output signal having a level proportional to the magnetic intensity of the analyzer. Programmable control circuitry 62 is responsive to the probe signal for controlling the regulator valve 56. It should be understood that illustration of a Hall probe 60 is purely exemplary since any suitable means may be employed which will enable the control circuitry 62 to follow the progress of the mass scan of the analyzer.

OPERATIONAL CONSIDERATIONS Referring to FIGS. 4 and 5, the clustering of argon is illustrated as effected by expansion by a nozzle havingan orifice diameter of 4 mils and an orifice thickness of mil. In FIG. 4 the source pressure is five atmospheres, and the numbers adjacent the peaks ranging from 1,160 to 1,400 are the molecular weights of the clusters of the respective peaks expressed in approximate atomic weight units. In FIG. 5, clustering is illustrated at three different pressures (two, three and five atmospheres) to show the effect of variation in source pressure on the formation of molecule clusters. The results of actual test data protrayed by FIGS. 4 and 5 were obtained under room temperature conditions (starting temperature of the gas), using a mass spec- .trometer with an ionizing electron energy of 50 electron volts. I

Since argon has a mass number of forty, conversion of the abscissa values in the graph of FIG. 5 to atomic weight units is readily affected by multiplying by a factor of 40. The ordinate values are plotted on a logarithmic scale to show the relative intensities of the ionized clusters. It may be appreciated that, for the particular nozzle configuration and starting temperature, the five atmospheres .of source pressure yields more uniform cluster intensities over possible ranges of interest, such as a mass range of approximately 1,200 to 1,400 amu (30 to 35 atoms in the cluster-ion). The strip chart recording reproduced in FIG. 4 shows the marker peaks that would be obtained over such range.

FIGS. 4 and 5, therefore, serve to illustrate several aspects of the practice of the present invention. First of all, of course, is the necessary selection of a marker gas capable of forming molecule clusters in the mass range of interest. Secondly, the source pressure (for a given nozzle configuration and temperature conditions) is preselected to cause the intensity of the molecule clusters to be relatively uniform throughout this mass range. Also, the mass intervals at which peaks are desired is a consideration affecting gas selection; in some instances, mixtures of gases may be desired in order to create a marker mass spectrum having particular mass interval combinations. In any event, however, the marker peaks occur at regular intervals determined by the basic molecule or molecules of the constituent gas or gases, thereby providing a predictable and reproducible spectrum.

The rare gases neon, argon, krypton and xenon are especially desirable due to their simple molecules and their ability to cluster readily in a controlled expansion. Furthermore, their relatively large negative mass defects make these gases especially desirable in high resolution analysis. Also, the inert chemical nature of these gases renders them compatible with mass spectrometer ion source components and the sample gas in instances where simultaneous detection of the marker and the sample isto be effected. However, it should be understood that other gases such as nitrogen and carbon dioxide may also be employed, depending upon the need to tailor the marker gas to the spectrum of the gas to be analyzed.

Referring to FIG. 6, this graph illustrates the change in the distribution of argon clusters caused by varying the nozzle configuration. The other parameters of the expansion were the same in all instances. A source pressure of three atmospheres was employed, the starting temperature was room temperature, and ionizing of the clusters was effected by a 50 ev electron field. The pictorial legend in FIG. 6 correlates the three curves with the three different nozzle configurations utilized.

For curve 70 in FIG. 6, the same nozzle configuration wasemployed as in the tests depicted in FIGS. 4 and 5. For curve 80, the argon gas was expanded through a 23 conical orifice having a thickness of mils and a diameter of 4.6 mils. For curve 90, argon was expanded ing to molecular weights over 1,000 amu. Usable clusthe scan is linear with respect to time and that the beam 16 is pulsed on for9 milliseconds and off for milliseconds, then the gas load is decreased by a factor of 10 and the mass marker is available 1,000 times (every mass unit) during the total scan time of 100,000 milliseconds. The intermittent operation is effected by the pulser 32 in conjunction with the electrically responsive valve whose armature 26 seats in therestriction 22 to close the nozzle 28 except when the coil 24 is energized by the pulser output. When the beam of neutral clusters reaches the electron field l8, ionization occurs and the cluster-ions exit from the ion source 14 through the port 46 to the analyzer of the spectrometer. It should be noted that the path of the beam 16 is entirely rectilinear, bends being avoided to prevent break up of the clusters.

The spectrum of the marker gas may be detected by the spectrometer separately from the sample gas, or the two may be introduced into the ion source 14 and detected simultaneously. In the former case, the separate recordings of the marker and sample gas spectra are compared, while in simultaneous detection the marker peaks occur in the spectrum of the sample but may be readily identified due to the regular mass intervals at which they occur, due to their mass defect, or due to an electrical signature given to the clusters by beam modulation (chopper disc 52).

Depending upon the mass range of interest and the marker gas being utilized, it may be desired to program the source pressure of the marker gas on a continuous basis during the mass scan. Referring to FIG. 2, the Hall probe 60 has'a DC output level proportional to the magnetic intensity of the analyzer, thus the programmable control circuitry 62 is able to respond as desired as the mass scan proceeds. For example, the circuitry 62 could be programmed to respond to increasing levels of the probe output (or one or more threshold levels) to increase the source pressure by actuation of the regulator valve 56 as the scan reaches higher masses in the range of interest where the intensity of the marker gas clusters would benefit from an increase in the source pressure. The net result is the maintenance of a more uniform intensity of the cluster-ions to optimize the intensity over the mass range.

For phase sensitive detection of the marker gas spectrum, the chopper disc 52 is employed to impart a unique signature to the cluster-ions. This further enhances the readability of the reference peaks of the marker gas spectrum since background and sample gas ions without the characteristic modulation are not significantly detected. The strip chart recording reproduced in FIG. 4 is an example of phase sensitive detection, and the sharp, well-defined peaks of the detected spectrum may be noted. This has particular importance in spectrometry since the reference markings must not only be predictable but also sharply defined and readily distinguishable from sample ions.

Having thus described the invention, what is claimed as new and desired to be secured by Letters Patent is:

1. In mass spectrometry, a method of marking molecular weights at regular mass intervals over a mass range within which ions of a gas to be analyzed are expected in order to provide a reference for identification of said ions, said method comprising the stepsof:

a. selecting a marker gas having molecules of known molecular weight and characterized by the capability of forming molecule clusters at said regular massintervals in a controlled expansion;

b. expanding said marker gas through a nozzle into a region of reduced pressure;

c. controlling the conditions under which said expansion is effected to cause partial condensation of the marker gas and attendant formation of a beam of molecule clusters having molecular weights distributed at said intervals in said mass range;

d. ionizing the molecule clusters of said beam; and

e. detecting the molecular weights of the ionized clusters and the molecular weights of the ions of the gas to be analyzed to thereby provide reference markings from which the ions may be identified.

2. The method as claimed in claim 1, wherein said step (c) includes preselecting the source pressure of said marker gas, the temperature thereof, and the configuration of said nozzle to effect said partial condensation and attendant cluster formation.

3. The method as claimed in claim 1, wherein said step (c) includes preselecting the source pressure of said marker gas, the temperature thereof, and the configuration of said nozzle to cause the intensity of the molecule clusters to be relatively uniform throughout said mass range.

4. The method as claimed in claim 1, wherein said step (c) includes varying the source pressure of said marker gas during detection of the molecular weights.

of the ionized clusters to optimize the intensity of the molecule clusters over said mass range.

5. The method as claimed in claim 1, wherein said detection of the molecular weights of the ionized clusters in said step (e) includes scanning said mass range in a mass spectrometer, and wherein said step (c) includes varying the source pressure of said marker gas during said scanning to optimize the intensity of the molecule clusters over said mass range.

6. The method as claimed in claim 1, wherein said step (b) includes intermittently delivering said marker gas to said nozzle to reduce the amount thereof subjected to expansion.

7. The method as claimed in claim 1, wherein is provided the additional step of modulating said beam of molecule clusters prior to detecting the molecular weights thereof in said step (e), whereby to impart a characteristic signature to the ionized clusters.

8. The method as claimed in claim 1, wherein said marker gas includes one of the rare gases selected from the group consisting of neon, argon, krypton and xenon.

9. In combination with a mass spectrometer having an ion source for receiving and ionizing a gas to be analyzed,'mass marker apparatus including:

means for supplying a marker under pressure having molecules of known molecular weight and characterized by the capability of forming molecule clusters at regular mass intervals over a preselected mass range in a controlled expansion of said marker gas;

a nozzle communicating with said supplying means for causing expansion of said marker gas as it passes therethrough into a region of reduced pressure; I

conduit structure having said nozzle disposed therein and presenting said region at the nozzle; and

means communicating with said structure for maintaining said region in an evacuated condition,

the pressure of said marker gas and the configuration of said nozzle being selected to cause partial condensation of the marker gas discharging from the nozzle and attendant formation of a beam of molecule clusters having molecular weights distributed at said intervals in said mass range,

said structure having outlet means communicating with said ion source to direct said beam into said source for ionization of the molecule clusters, whereby to provide the spectrometer readout with reference markings for identification of the ions of the gas to be analyzed. I

10. In the combination as claimed in claim 9, wherein said spectrometer scans said mass range in the detection of the molecular weights of ions emanating from said ion source, and wherein said apparatus further includes means coupled with said supplying means and responsive to the progress of the scan of said mass range for varying the supply pressure of said marker gas to optimize the intensity of the molecule clusters over said mass range. v

11. The apparatus as claimed in claim 9, wherein an electrically responsive valve is provided in association with said nozzle for controlling the dischargeof said marker gas therefrom, and means coupled with said valve for exciting the latter with time-spaced electrical pulses to cause intermittent operation of the valve and attendant reduction of the amount of said marker gas discharged from the nozzle, thereby minimizing the gas load.

12. The apparatus as claimed in' claim 9, wherein beam into said ion source along a rectilinear path. 

1. In mass spectrometry, a method of marking molecular weights at regular mass intervals over a mass range within which ions of a gas to be analyzed are expected in order to provide a reference for identification of said ions, said method comprising the steps of: a. selecting a marker gas having molecules of known molecular weight and characterized by the capability of forming molecule clusters at said regular mass intervals in a controlled expansion; b. expanding said marker gas through a nozzle into a region of reduced pressure; c. controlling the conditions under which said expansion is effected to cause partial condensation of the marker gas and attendant formation of a beam of molecule clusters having molecular weights distributed at said intervals in said mass range; d. ionizing the molecule clusters of said beam; and e. detecting the molecular weights of the ionized clusters and the molecular weights of the ions of the gas to be analyzed to thereby provide reference markings from which the ions may be identified.
 2. The method as claimed in claim 1, wherein said step (c) includes preselecting the source pressure of said marker gas, the temperature thereof, and the configuration of said nozzle to effect said partial condensation and attendant cluster formation.
 3. The method as claimed in claim 1, wherein said step (c) includes preselecting the source pressure of said marker gas, the temperature thereof, and the configuration of said nozzle to cause the intensity of the molecule clusters to be relatively uniform throughout said mass range.
 4. The method as claimed in claim 1, wherein said step (c) includes varying the source pressure of said marker gas during detection of the molecular weights of the ionized clusters to optimize the intensity of the molecule clusters over said mass range.
 5. The method as claimed in claim 1, wherein said detection of the molecular weights of the ionized clusters in said step (e) includes scanning said mass range in a mass spectrometer, and wherein said step (c) includes varying the source pressure of said marker gas during said scanning to optimize the intensity of the molecule clusters over said mass range.
 6. The method as claimed in claim 1, wherein said step (b) includes intermittently delivering said marker gas to said nozzle to reduce the amount thereof subjected to expansion.
 7. The method as claimed in claim 1, wherein is provided the additional step of modulating said beam of molecule clusters prior to detecting the molecular weights thereof in said step (e), whereby to impart a characteristic signature to the ionized clusters.
 8. The method as claimed in claim 1, wherein said marker gas includes one of the rare gases selected from the group consisting of neon, argon, krypton and xenon.
 9. In combination with a mass spectrometer having an ion source for receiving and ionizing a gas to be analyzed, mass marker apparatus including: means for supplying a marker gas under pressure having molecules of known molecular weight and characterized by the capability of forming molecule clusters at regular mass intervals over a preselected mass range in a controlled expansion of said marker gas; a nozzle communicating with said supplying means for causing expansion of said marker gas as it passes therethrough into a region of reduced pressure; conduit structure having said nozzle disposed therein and presenting said region at the nozzle; and means communicating with said structure for maintaining said region in an evacuated condition, the pressure of said marker gas and the configuration of said nozzle being selected to cause partial condensation of the marker gas discharging from the nozzle and attendant formation of a beam of molecule clusters having molecular weights distributed at said intervals in said mass range, said structure having outlet means communicating with said ion source to direct said beam into said source for ionization of the molecule clusters, whereby to provide the spectrometer readout with reference markings for identification of the ions of the gas to be analyzed.
 10. In the combination as claimed in claim 9, wherein said spectrometer scans said mass range in the detection of the molecular weights of ions emanating from said ion source, and wherein said apparatus further includes means coupled with said supplying means and responsive to the progress of the scan of said mass range for varying the supply pressure of said marker gas to optimize the intensity of the molecule clusters over said mass range.
 11. The apparatus as claimed in claim 9, wherein an electrically responsive valve is provided in association with said nozzle for controlling the discharge of said marker gas therefrom, and means coupled with said valve for exciting the latter with time-spaced electrical pulses to cause intermittent operation of the valve and attendant reduction of the amount of said marker gas discharged from the nozzle, thereby minimizing the gas load.
 12. The apparatus as claimed in claim 9, wherein chopper means is provided in the path of said beam for modulating the latter, whereby to impart a characteristic signature to the molecule clusters.
 13. The apparatus as claimed in claim 9, wherein said nozzle and said structure are arranged to direct said beam into said ion source along a rectilinear path. 